Sunday, May 24, 2009

One kilogram does not equal 2.2 pounds

How you react to this statement tells me something, I think. If you've taken physics, you know that pounds are a unit of force, while kilograms are a unit of mass, and therefore, 2.2 pounds does not equal 1 kilogram. 1 kilogram of matter, on Earth, weighs 2.2 pounds (give or take a few small fractions; Earth's gravity field isn't quite uniform); on the moon, the same object will still mass one kilogram, while it will weigh only a few ounces.

So it's quite technically correct to say 1 kilogram doesn't equal 2.2 pounds. On the other hand, for all practical intents and purposes, that's the useful conversion to make, since the newton (metric unit of force) and the slug (standard imperial unit of mass) are more rarely used units.

Some people react with a nod. They're aware of the difference, and consider it an important one. Others react with a groan - they know that technically it's correct, but as far as they're concerned, the difference is a technical distinction that doesn't really matter. And a few become quite confused, because they don't know what the distinction is between weight and mass.